Integrate Key module for secure DeepL API key management

Problem/Motivation

Currently, DeepL API keys are stored in configuration, which can inadvertently be exposed in version-controlled configuration files. This poses a security risk.

Proposed resolution

Integrate the Key module to allow administrators to store DeepL API keys securely using the Key module’s storage system.

Remaining tasks

• Update TMGMT DeepL configuration form to include a Key module field.
• Modify DeepL API client to retrieve the key from the Key module.
• Add dependency on the Key module in tmgmt_deepl.info.yml.
• Update existing documentation and explain how to handle key module
• Update existing unit tests

User interface changes

Add a dropdown field in the TMGMT DeepL configuration form to select a key from the Key module.

API changes

None.

Data model changes

None.